As the sizes and spaces of structures in a semiconductor structure decrease, it becomes harder to form the structures arranged densely in parallel. One of the reasons is the resolution limitation of lithography processes. To solve this problem, multiple patterning technologies, such as double patterning technology, are proposed. An example is pitch splitting. The structures arranged densely in parallel are divided into two interleaving sets, and the two sets of the structures are formed using two separate masks. More specifically, one half of the structures are formed using one of the masks. The other half of the structures then are formed using the other one of the masks in a manner that each of these structures is located between two previously formed structures. As such, the structures arranged densely in parallel can be formed by less densely arranged patterns on the masks, wherein the pitch of which is twice of the pitch of the structures to be formed.